1. Technical Field of the Invention
The present invention relates to a fishway to bypass a vertical descent in a flowing watercourse with an upstream water inlet and a downstream water outlet and with basins arranged between them substantially in a downstream direction, each of which has an inflow slot and an outflow slot as vertical through-flow slots and deflection means to form a meandering passage.
2. Prior Art
Fishways, particularly fish passes and fish ladders, enable migrating salmonidae and small biocenoses to bypass dams, thereby restoring the passableness of bodies of water.
From U.S. Pat. No. 132 349, a fishway according to the preamble is known, which is made of substantially rectangular basins arranged on a sloped ramp with deflection walls formed inside the basins. To create a meandering passage, the through-flow slots in the individual basins are provided alternately in opposite deflection walls and in the cross walls that separate the individual basins. This design has the disadvantage that the basin walls have sharp edges in the areas of the through-flow slots, which cause the steady flow to be disrupted at these locations, so that the fish who are migrating upstream become disoriented and get diverted to the water surface by the turbulent flows and secondary flows and potential change in the curvature path of the water flow in the area of the through-flow slot. The fish are not oriented towards the bottom when they migrate upstream, and they depart from the flow rate of the flow profile that is ideal for their particular species.
Furthermore, dead spaces form in the comers of the basins, in which disadvantageous deposits of detritus accumulate that result in colmations and cause a considerable amount of maintenance work.
Since the ramp has an almost smooth surface, the dam in a flowing watercourse represents an unsurmountable obstacle for small fish and benthic organisms despite the fishway, because the flow rate near the bottom is too high.
In a technical essay ("Wasserwirtschaft 79 (1989) 2, page 67") a fishway--the so-called vertical-slot pass--is described, which consists of rectangular basins, which are arranged successively in the direction of the through-flow and whose cross walls have vertical through-flow slots. The through-flow slots are all formed adjacent to the same longitudinal side of the system, which causes a highly irregular flow pattern (see FIG. 4b, page 68). Turbulent flows, which have a disorienting effect on the fish, are present both in the inlet area into the basins and in the outlet area from the basins.
Because of their shape, the basins in the vertical-slot pass also have dead spaces where material is deposited, which can only be removed with complicated, time-consuming cleaning efforts.
To allow smaller fish to migrate upstream, the vertical-slot pass has a basin floor with a rough texture, which reduces the flow rate in the bottom area.
Practice has shown, however, that loose stones get stuck in the through-flow slots and block the passage, with the result that the creatures are impeded in their migration upstream.
It is true that the vertical-slot pass is an improvement over conventional fishways such as rhomboid passes, or the like regarding the passableness for migrating salmonidae and benthic organisms. However, it is not an ideal system under fluidics aspects since a basin shape with comers not only furthers the formation of dead spaces but also the development of turbulent secondary flows, which are augmented further by sharp edges in the area of the through-flow slots.
Various fishways with round basins (the so-called round-basin passes) are furthermore known from a technical essay ("Wasser & Boden 47, (1995) 3, p. 55 ff"), in which the round basins either have alternating pairs of cut-outs in the crest and base in the direction of the slope, or through-flow slots. These openings have sharp edges that result in the above shortcomings, such as a detachment of the steady flow and the formation of turbulent flows. The flow does not run parallel to the base of the basin but is directed upward so that the fish who are migrating upstream are receiving the message to swim towards the water surface instead of remaining oriented towards the bottom.
The alternating orientation of the openings in the direction of the slope results in the formation of an undulating main flow in the sequentially traversed basins, whereby this main flow runs continuously in the direction of the slope and accordingly flows very fast, at times exceeding the critical flow rates for small fish. To alleviate this problem, the slope is reduced to approximately 10% which, however, entails a disadvantageous increase in the number of required basins.
In the case of basins with a through-flow slot, the steel elements making up the basin must be stabilized in the area of the through-flow slot with welded-in cross struts. This has the disadvantage that the flow is divided and disorienting vortexes are created.
Round-basin passes must therefore be assembled on-site, which is both time consuming and expensive.
Generally known are fishways whose rectangular basins have cut-outs in the crest and holes for the creatures to slip through. Practice has shown that many of these systems are not functional. The cause lies in design errors and in the labor-intensive maintenance of these systems. A further shortcoming lies in the fact that systems of this type only permit the passage of certain species of fish, as the maximum velocities for certain species of fish are exceeded at the slip-though holes. Small creatures cannot pass fish passes of this type. The basin passes with slip-through holes and crest cut-outs require a considerable amount of maintenance since the slip-through holes and crest cut-outs become clogged with flotsam, or the basins become filled with mud which, in the worst case, can make them impassable. The natural self-cleaning effect of these types of systems greatly depends on the flow rate and amount of water. The operability of this fishway is not guaranteed at low water levels as these systems require a great amount of water.
The problem of the passableness of flowing watercourses has also been addressed with an artificial creek, which is designed based on a natural creek and leads around the dam. This solution requires significant amounts of available space and water. Also, accompanying landscaping measures need to be carried out after the construction phase, resulting in considerable expenditures.